LiMnPO4, which is one of olivine-type lithium metal phosphates, is cheaper than LiCoO2, which has been widely used as a cathode-active material for a lithium secondary battery, and has higher energy density than that of LiFePO4, which is an iron-based olivine-type lithium metal phosphate. Therefore, LiMnPO4 is expected to be a cathode-active material for a lithium secondary battery, in particular, a large-sized lithium secondary battery for a vehicle.
As a method for producing LiMnPO4, solid phase synthesis (Patent Document 1), hydrothermal synthesis (Patent Document 2 and Non-Patent Document 1), and a sol-gel method (Patent Document 2) are known. Among these methods, a hydrothermal synthesis which can produce LiMnPO4 having a small particle diameter at relatively low temperatures within a short time is considered to be the best method.